1. Field of the Invention
The present invention generally relates to a method and apparatus for equipping a two-stroke internal combustion (ic) engine of the type having reciprocating pistons and self supercharging by a post-charging effect, and in particular, although not exclusively, an engine having at least one single-acting cylinder.
2. Prior Art
In the prior art, a large number of proposals already exist for using the direct action of the gases leaving a cylinder to compress and introduce a reserve of air into a cylinder at an appropriate moment in the cycle in order to supercharge it.
Most of these proposals make use of the direct action of the exhaust gases from one cylinder, and vice versa, with the angular phase difference between the cylinders communicating in this way being judiciously selected (see Swiss patent No. CH 593.420).
Proposals for making the energy of exhaust gases in a cylinder act directly on an air reserve for improving the charging and efficiency of the same cylinder are much rarer, particularly if, for obvious reasons of efficiency, account is taken only of those proposals which enable the phases of transferring pressure from the cylinder to the air reserve and then to the cylinder to take place in a closed space within the system under consideration, in order to avoid any loss of energy to the outside.
For reasons of convenience, the set of components constituting the above-mentioned system is referred to below as a "compressor system".
In general, the term "scavenging air" is used below to specify the scavenging agent used for renewing the burnt gases in a cylinder, regardless of whether it is constituted by pure air or by some other mixture of fuel and oxidizer.
The number of proposals is even smaller if account is taken solely of those which provide a post-charging effect as opposed to a compression of the entire gas quantity admitted to the cylinder (which is difficult to achieve in a two-stroke cycle unless the exhaust back pressure is simultaneously increased).
The term "post-charging" is used to mean injecting an additional quantity of air into the cylinder after scavenging, and after the exhaust ports have been closed.
The advantage of post-charging has been known for a long time, in particular, for use in the two-stroke cycle.
For a given amount of external compression work applied to the quantity of air, post-charging provides an increased overall charging by raising air purity. This is because the mass of residual gases not expelled at the end of the scavenging phase remains unaltered during the post-charging phase and therefore becomes relatively smaller compared with the total mass of air contained in the cylinder after the post-charging effect. This is equivalent to improving the scavenging efficiency and to making better utilization of the scavenging air.
One such apparatus for a single cylinder post-charging two-stroke engine is described in U.S. Pat. No. 1,362,080 and is shown in FIG. 5 thereof.
In said apparatus, the compressor system comprises a supply of air external to the engine and connected at one of its ends to a volume constituting an anti-chamber to said air reserve and to a duct communicating with a source of scavenging air (via a three-way valve), and at its other end to the admission pipe leading to the admission port of the cylinder (controlled by fluid-flow control means, e.g. of the valve type, housed in the cylinder head), and to a first exhaust duct for evacuating the exhaust gases to the outside (again via a three-way valve), after they have been used.
The volume forming an anti-chamber to the air reserve is itself connected at its upstream end to a non-return valve located immediately downstream from a pre-exhaust opening connected to a pre-exhaust port (controlled by an opening provided in the liner) which is distinct from the exhaust port per se (also controlled by an opening provided in the liner) opening out to a second exhaust duct which is the exhaust duct from the cylinder.
Following the various sequences of the compressor system in cyclic order, the exhaust gases which, during the pre-exhaust phase (when the system formed by the cylinder and the anti-chamber is closed to the outside, the cylinder's exhaust port and the three-way valve downstream from said volume both being closed) escape from the cylinder via the pre-exhaust port and are stored under pressure in said volume while waiting to be transferred and hence transmit their pressure energy to the air previously stored in said reserve. The presence of the non-return valve makes it possible to maintain the pressure of the exhaust gases in said volume forming the anti-chamber.
In the following post-charging phase, the exhaust gases penetrate into the reserve (which at that moment is closed to the source of scavenging air and the first exhaust duct, but is in communication at its upstream end with said volume defining the anti-chamber, and at its downstream end with the cylinder admission pipe, again by virtue of each of the two above-mentioned three-way valves being suitably set), thereby compressing the air previously stored therein and urging it into the cylinder which then communicates with its admission pipe.
The compressor system cycle terminates with the scavenging period of the air supply in order to replace the burnt gases left therein after the post-charging phase with fresh air, again by setting the above-mentioned three-way valves to cause said reserve to communicate with the source of scavenging air at its upstream end and with the first exhaust duct at its downstream end (while simultaneously being closed relative to the anti-chamber volume and the admission pipe to the cylinder). It may be observed that more time is available for scavenging the air reserve which takes place during compression, combustion and expansion within the cylinder than for scavenging the cylinder per se.
It may also be observed, with particular reference to the compressor system and the admission phase of the engine cycle, that the main flow direction of the exhaust gases, of the air injected into the air reserve, and of the air injected into the cylinder is uni-directional, i.e. along the same direction from the pre-exhaust port to the downstream end of the air reserve situated in the vicinity of the cylinder admission pipe.
Although such apparatus implements a method corresponding to an ideal thermodynamic cycle, it suffers from the following drawbacks, in particular:
a) The external position of the air reserve and of the volume forming anti-chamber for the exhaust gases, and the need to provide three additional valves in order to provide fluid flow control timing during the compressor system cycle and the charging part of the engine cycle give rise to an assembly which is very complex and bulky. This mechanical complexity is difficult to justify, in particular for an application with multicylinder engines and in the current technical context where turbo-charging has become widespread.
b) These additional valves are either automatically actuated in which case they present a degree of unreliability when located in a flow of high temperature gas, or else they are mechanically actuated. The latter type gives rise to difficulties both for lubrication and as regards the speed at which they open and close. Moreover, they must have sufficient flow section areas to implement the gas exchange diagrams required by the compressor cycle and the charging part of the engine cycle.
c) The position of the additional valves necessarily gives rise to a certain number of dead zones since it is not always possible to control the exact position of the gas-air interface. This means either that burnt gases are admitted into the cylinder at the end of the post-charging phase or else that fresh air is lost during the subsequent scavenging period of the air reserve.